Unlike private networks such as PBXs, public telephone networks operate on a connection-oriented basis. Before the actual data exchange between the subscribers, connection setup takes place in which the path through the network is specified and the required resources such as circuits, channels and bandwidths for these connections are reserved. The path through a communication network is specified within the switching system on the basis of the address supplied at connection setup and stored traffic routing data. At connection setup, the most favorable path for the connection through the network from the A-subscriber to the B-subscriber is determined according to pre-defined algorithms. After connection setup, the B-subscriber is called. The required or agreed bandwidth is possibly monitored during the connection to prevent overloads in the switches involved and in order not to appreciably limit the quality of the other connections.
Analogously to the reserving of channels and circuits in connection-oriented communication networks such as ATM networks (Asynchronous Transfer Mode), in connectionless networks such as IP networks (Internet Protocol) transmission and routing capacity is reserved for the required connection. This is necessary in such networks in order to be able to guarantee a specified QoS (Quality of Service) for the communication.
Multimedia services such as videophone and videoconference require broadband connections with bit rates of 1.5 Mbit/s to 100 Mbit/s or more. Connections of this kind generally contain a plurality of user data streams. Thus an audio and a video data stream are generally required for videotelephony, as the audio and video data are compressed using different compression methods with different compression rates, the video data stream requiring a higher bandwidth than the audio data stream despite compression.
If multimedia connections of this kind are established according to the procedure outlined above, which is described in Kalmanek, C. R.; DOSA: An Architecture for Providing a Robust IP Telephony Service, AT&T Labs, New Jersey, for Internet telephony, network resources are wasted, as the full bandwidth is reserved in the network or the broadband connection is already in place during calling of the B-subscriber (ringing of the videophone at the receive end). The network operator receives no revenue for this service, as charges are not payable until the B-subscriber accepts the call and the connection is put into operation. Likewise the case may arise that resources requested by the A-subscriber for setting up the broadband connection are not available to the network, the connection is not therefore established and the B-subscriber is not even called in the first place. This means lost revenue for the network operator and increasing customer dissatisfaction.
One approach for solving the problem is known from Sinnreich, H.; Interdomain IP Communications with QoS, Authorization and Usage Reporting, Internet Draft, Draft-sinnreich-interdomain-sip-qos-01.txt, 2000, in which the B-subscriber is called even though the connection is not yet stable in the network or the resources requested by the A-subscriber are not yet available.
However, it may happen that, when the B-subscriber goes off-hook, the connection is set up with a delay, with poor or not the desired quality, or not at all. In addition, ITU-T Recommendation Q.2962 of the International Telecommunication Union—Telecommunication Standardization Sector describes broadband ISDN signaling protocols that allow an A-subscriber to initiate the conversion of a minimum connection to a broader-band connection. According to this approach, the A-subscriber already possesses a minimum connection when he initiates the conversion. However, the A-subscriber cannot be prevented from requesting a broadband connection from the very outset, which means that the original situation with the associated problems described above can therefore arise. According to ITU-T Recommendation Q.2962, the A-subscriber can specify, to the network or his switch, one or more alternatives for establishing the connection. Thus, if the resources are available, a broadband connection is established. However, if network resources are again reserved during calling of the B-subscriber, and if no resources are available for a broadband connection, only a minimum connection is established which is not subsequently converted to a broadband connection.